U.S. attorney Preet Bharara said the agent admitted to being a significant source of information to reporters at both The New York Times and The Wall Street Journal, resulting in some seven articles published between May 30, 2014 and June 23, 2014. Those articles, which contained "sensitive and confidential details, including trades being examined, records being analyzed, the name of an individual approached by the FBI and the supposed targets of the investigation," led to an indictment of Walters and Dean Foods former executive Thomas C. Davis.

"The agent further admitted that, prior to his Dec. 6 interview, he had hidden those communications with the press from both the U.S. Attorney's Office and others within the FBI," Bharara said.

The letter states that the agent has been reported to the FBI's Office of Professional Responsibility, and that the issue has been brought to the Office of Inspector General for the Department of Justice -- but the disclosure of said confidential information could have violated the criminal procedure of the case.

While Mickelson was not charged, he did agree to repay just over $1 million in "ill-gotten gains" from stock trades he and Walters made on Dean Foods in 2011. Walters trial is set to start July 29, 2017, in New York.

Phil Mickelson waits on the fourth hole during the second round of the 2016 U.S. Open at Oakmont Country Club.